Polycarbonates have excellent mechanical properties such as impact resistance, as well as excellent heat resistance, transparency and other properties. They are widely used in applications such as various types of mechanical components, optical disks, and automotive parts.
Polycarbonates such as these have hitherto been prepared by a method involving the direct reaction of an aromatic dihydroxy compound such as bispherol A with phosgene (interfacial process) or a method involving the transesterification of an aromatic dihydroxy compound and a diester carbonate (melt process).
Of these two methods, the melt process has the advantage of being able to prepare polycarbonates more inexpensively than the interfacial process. Moreover, because it does not use toxic substances such as phosgene, it is also desirable from the standpoint of environmental health.
In polycarbonate production methods by prior-art melt processes, alkali metal compounds, alkaline earth metal compounds or the like are generally used as the catalyst in an amount of 10.sup.-3 to 10.sup.-6 mole per mole of the aromatic dihydroxy compound.
However, polycarbonates obtained using an amount of catalyst such as this have a poor residence stability. That is, when the polycarbonate is melt-molded, some of it will at times undergo thermal decomposition, a decline in molecular weight, discoloration, and a decrease in transparency.
For example, Japanese Published Examined Patent Application [Kokoku] No. 54-44,303 (1979) discloses a method for obtaining polycarbonates that prevents a decline in the molecular weight at the time of molding. This method involves carrying out a polycondensation reaction using an amount of 3.7.times.10.sup.-6 mole of the sodium salt of bisphenol A as the catalyst per mole of the bisphenol A serving as the reactive monomer, and adding methyl benzenesulfonate to the polycarbonate thus obtained. However, the polycarbonate obtained by the method cited in this Kokoku has a poor initial color tone, readily undergoes yellowing at the time of melt molding, and tends to have an inferior heat resistance. Moreover, the water resistance is not enhanced, hydrolysis readily arises, and there is a tendency for the transparency to decrease.
Hence, the appearance of a method for preparing polycarbonates that is able to stably prepare polycarbonates having excellent residence stability such as hue stability and heat stability at the time of melt molding, as well as excellent water resistance and transparency, has been awaited.
The inventors conducted research on such polycarbonate production methods, as a result of which they found that by making alkali metal compounds and/or alkaline earth metal compounds present in a specific amount within the polycondensation reaction system and by adding a specific sulfonic acid compound to the polycarbonate obtained, polycarbonates can be prepared which have excellent residence stability such as hue stability and heat stability at the time of melt molding, and in particular excellent water resistance and transparency.